What is unsteady flow process????

In the previous sections, it was noted that nozzles, diffusers, turbines, compressors and other devices undergo a steady-flow process because of their long-time running consideration. But their startup and shutdown periods undergo transient operations since their states change with time. The flow processes involved are called unsteady-flow processes, or transient-flow processes. Unlike steady-flow processes, unsteady-flow processes start and end over some finite time period (Δt). An additional example of an unsteady-flow process is filling or discharging a tank.During an unsteady-flow process, the mass in the control volume changes with time. 

Example unsteady flow process